Method of encoding data in a monochrome media

ABSTRACT

A method for encoding data in a monochrome media utilizing the capability of the media for grayscale resolution. A document is processed to provide an image in electronic format, in which each pixel has an assigned data value with a given bit depth. A mapping operation is performed for generating a monochrome data word ( 120 ), preferably having a reduced bit depth. The monochrome data word ( 120 ) can be used to encode multiple data fields ( 114, 116, 118 ). A printer ( 92 ) then produces a preserved document record ( 90 ) in which the appropriate monochrome data word ( 120 ) determines the grayscale value for each pixel.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] Reference is made to commonly-assigned copending U.S. patentapplication Ser. No. 10/000,407, filed Nov. 2, 2001, entitled DIGITALDATA PRESERVATION SYSTEM, by Wong et al., the disclosure of which isincorporated herein.

FIELD OF THE INVENTION

[0002] This invention generally relates to a method for long-termpreservation of data and more particularly relates to preservation ofdata associated with an image on monochrome media.

BACKGROUND OF THE INVENTION

[0003] In spite of numerous advances in development and use of colorimaging media, there are a number of conditions in which monochromeimaging media must be used. For example, archival or long-termpreservation of images may require that images be stored on a monochromemedia. As another example, there can be advantages to compact storage ofimages, where it is desirable to use a monochrome media for preserving acolor image, with accompanying encoded information.

[0004] There can be a considerable amount of data associated with animage, where the data concerns the image itself. For example, inprinting applications information about an image can include colorseparation data for corresponding cyan, magenta, yellow, and black(CMYK) inks or other colorants. Typically, color separations can bestored as separate images on monochrome media, so that each colorseparation is then stored as a separate monochrome image. For example,U.S. Pat. No. 5,335,082 (Sable) discloses an apparatus using a pluralityof monochrome images as separations of a composite color image.Similarly, U.S. Pat. No. 5,606,379 (Williams) discloses a method forstoring color images on a monochrome photographic recording medium inwhich separate R, G, and B or lightness and chroma channels are storedas separate images. Such methods may be acceptable for some types ofstorage environments, however, it can be appreciated that there would beadvantages in storing fewer images and in providing a more compactarrangement.

[0005] A number of existing methods for encoding data associated with animage are directed to the problem of encoding color image informationwithin a monochrome image. Examples of solutions for this type ofimage-data encoding include the following:

[0006] U.S. Pat. No. 5,557,430 (Isemura et al.) discloses a method forprocessing a color image in order to encode color recognition data on aresulting monochrome image. The method described in U.S. Pat. No.5,557,430 provides some amount of color information available; however,such a method is usable only in limited applications, such as where onlya few spot colors are used on a document, such as a businesspresentation.

[0007] U.S. Pat. No. 5,701,401 (Harrington et al.) discloses a methodfor preserving the color intent of an image when the image is printed ona monochrome printer. Distinctive patterns are applied for each colorarea.

[0008] U.S. Pat. No. 6,179,485 (Harrington) discloses a method forencoding color information in monochromatic format using variouslystroked patterns. This method is primarily directed to preserving colorintent for fonts and vector (line) drawings. Similarly, U.S. Pat. No.6,169,607 (also to Harrington) discloses methods for encoding color datain monochrome text using combinations of bold, outline, and fill patterneffects. U.S. Pat. Nos. 4,688,031 and 4,703,318 (both to Haggerty)disclose methods for monochromatic representation of color usingbackground and foreground patterns.

[0009] Overall, the methods disclosed in U.S. Pat. Nos. 5,557,430;5,701,401; 6,179,485; 4,688,031; and 4,703,318 may provide some colorencoding that is useful for documents using a very limited colorpalette, such as business documents and charts. However, these methodswould be unworkable for a full-color image, where the need for apixel-by-pixel encoding would require considerably greater spatialresolution than these methods provide. At best, such methods may be ableto provide a rudimentary approximation of color using relative lightnesslevels. However, there is no provision in any of the schemes given inthe patents listed above for encoding of additional data related to thecolor image when it is represented in monochrome format.

[0010] Known methods used for encoding data associated with an imageinclude that disclosed in U.S. Pat. No. 5,818,966 (Prasad et al.), whichdiscloses encoding color information along a sidebar that prints with amonochrome version of a document. This solution would have only limitedvalue, such as with charts and other business graphics using a palettehaving a few colors.

[0011] Each of the solutions noted above is directed to encoding dataabout the image itself, such as color data. However, it may be useful toencode other types of data that, although not directly concerned withimage representation itself, may be associated with an image. Forexample, an image can have associated audio data, animation data,measurement data, text, or other data, where it is advantageous to havesuch data coupled in some manner with the image. Use of a sidebar, suchas disclosed in U.S. Pat. No. 5,818,966 provides some solution, however,such a solution requires additional media area that may not beinherently coupled to an image. Because most images are stored in arectangular format, any additional patch of information must be storedabove, below, or on either side of the image. Accompanying informationwould take up additional space on the media. In addition, any encodedinformation provided in a separate area of the storage medium could beintentionally or unintentionally separated from the image itself.

[0012] Methods for encoding data in visible form on a monochromaticmedium include the following:

[0013] U.S. Pat. No. 5,091,966 (Bloomberg et al.) discloses the use ofmonochromatic glyph codes encoded onto a document image, in visualjuxtaposition to the image. Notably, the area in which the glyph codesare encoded is separate from the document image itself with thissolution.

[0014] U.S. Pat. Nos. 6,098,882 (Antognini et al.) and 4,939,354 (Priddyet al.) disclose methods for encoding digital data onto paper in compactform using bi-tonal markings grouped in a spatial array of cells. Theability to provide increasingly more compact data storage on monochromemedia, using methods such as those disclosed in U.S. Pat. Nos. 6,098,882and 4,939,354, can be attributed, in large part, to continuingimprovement in the spatial resolution of desktop scanners.

[0015] U.S. Pat. No. 5,278,400 (Appel) discloses a method for encodingdata in a cell comprising multiple pixels, where the halftone gray levelof each individual pixel, in combination with other pixels within thecell, encodes a data value for the cell. The method disclosed in U.S.Pat. No. 5,278,400 also takes advantage of increased spatial resolutionof scanners, supplemented by the capability of a scanner to sense graylevel at an individual pixel within a cell.

[0016] The methods disclosed in U.S. Pat. Nos. 5,278,400; 6,098,882; and4,939,354 provide data encoding for compact data storage on a monochromemedium.

[0017] However, neither these methods, nor the methods disclosed in thepatents cited above provide a mechanism for integrally coupling data toan associated image. These methods also require space on the monochromemedium, in addition to that required for the image itself.

[0018] Some types of monochrome media, such as paper, for example, allowreproduction of only a limited range of perceptible densities. That is,only a few different density levels can be reliably printed or scannedfrom such types of media.

[0019] However, there are other types of monochrome media that havepronouncedly greater sensitivity. Conventional black and whitephotography film, for example, is able to faithfully and controllablyreproduce hundreds of different gray levels, each measurably distinct.Other specialized films and photosensitive media have been developedthat exhibit wider overall dynamic range and higher degrees ofresolvable density, able to produce a higher number of distinctgrayscale values.

[0020] It is instructive to observe that the term “grayscale” isconventionally associated with a range of densities where themonochromatic color hue is black. However, for the purposes of thisapplication, the monochromatic color hue, or color base, for a grayscaleimage need not be black, but could be some other color. For example,some types of monochrome film have a very dark blue color hue that couldbe used as the color base for grayscale imaging. Regardless of theprecise color hue, the term “grayscale” as used herein relates to arange of measurable density values of a single base color, formed atindividual pixel locations on a digital preservation medium.

[0021] It is instructive to note that the human viewer perceives only alimited number of grayscale gradation values, centered on a range thatis well within the overall dynamic range of most types of photosensitivemedia. Generally, a bit depth of 8-bits is sufficient for storing thegrayscale values perceptible in monochrome images. While, for humanperception, there may be no need for visible representation exceeding abit depth of 8-bits, it could be possible to reproduce an image having alarger bit depth, with 10, 12, or greater bits of resolution, forexample, using photosensitive media described above. In fact, manyconventional scanners have additional sensitivity for grayscaleresolution. The four-color printing industry, for example, useshigh-resolution color scanners that are able to provide very highspatial resolution and very sensitive color resolution. As just oneexample, the SG-8060P MarkII High-end Input Scanner from DainipponScreen claims to be capable of scanning at 12,000 dpi and providing48-bit RGB resolution. Anticipated improvements in scanning technologyare expected to make the capability for such high resolution and highdensity sensitivity more readily accessible and more affordable. Thiswould mean, for example, that a scanner could have sufficientsensitivity to provide data with a bit depth exceeding 8-bits whenscanning a highly sensitive media, even though 8-bit grayscalerepresentation is sufficient for storing an image in human-readableform.

[0022] Conventionally, in converting a full-color image to a monochromeformat only the relative lightness or darkness value of a color is usedto determine a corresponding grayscale representation. Chromainformation, which indicates color hue content, is largely ignored. Forthis reason, restoration of original color information to an image, onceconverted to monochrome format, is not easily feasible. It can beappreciated that image storage solutions that preserved some colorinformation, even if approximate, could be advantageous.

[0023] Thus it can be seen that conventional document storage andpreservation solutions fall far short of meeting the need to integrallycouple data related to an image to the image itself. Even though thecapability exists for reproducing and measuring image densitysensitivity well in excess of the human-perceptible range, no use hasbeen made of this excess capability for its data storage potential.

SUMMARY OF THE INVENTION

[0024] It is an object of the present invention to provide a method ofencoding, in a monochrome medium, data about a document that has beenreceived in electronic form. Briefly according to one aspect of thepresent invention the method comprises:

[0025] (a) converting the document to a rasterized image in which eachpixel is assigned a raster value;

[0026] (b) for each pixel:

[0027] (b1) assigning a data word having a predetermined bit depth, thedata word comprising a first data field and a second data field;

[0028] (b2) encoding a first component of the raster value into thefirst data field;

[0029] (b3) encoding a second data value into the second data field;

[0030] (b4) generating a grayscale data value comprising the first datafield and the second data field;

[0031] (b5) forming, onto the monochrome medium, a grayscale pixelconditioned by the grayscale data value; and

[0032] thereby encoding data about the document in the monochromemedium.

[0033] It is a feature of the present invention that it allows acoupling of data associated with a document to the document itself, insuch a way that the coupled, encoded data is not easily separable fromthe image of the document, but does not obscure the image. At the sametime, the coupled data can be encoded in a manner that is imperceptible,while the document itself is visible. The method of the presentinvention allows a document and its associated encoded data to bepreserved on a monochrome preservation medium, available for futureaccess and decoding.

[0034] The present invention takes advantage of the high levels ofresolvability available with some types of monochromatic media.High-resolvability allows encoding of data in gray levels, where thenumber of gray levels that can be reproduced exceeds the number ofdistinct gray levels that can be distinguished by the human eye.

[0035] It is an advantage of the present invention that it provides amethod for long-term preservation of a document and its associated dataas a single unit.

[0036] It is yet a further advantage of the present invention that itprovides a method for preserving, onto a monochrome medium, data about afull-color image.

[0037] It is yet a further advantage of the present invention that itprovides a method for storing metadata associated with a document orwith document image processing in a manner such that the metadata isclosely coupled or, in some embodiments, integrally coupled to thedocument.

[0038] It is yet a further advantage of the present invention that itprovides a method for storage of data having considerable density, yetwithout making existing equipment obsolete. That is, existing imagesensing apparatus may not be able to take advantage of denser dataencoding capabilities offered by the present invention, but can still beused for scanning an image preserved using these techniques, forexample. For images, higher order density values typically store thelightness channel information, so that an image remains human-readableeven if it contains considerable additional data content.

[0039] These and other objects, features, and advantages of the presentinvention will become apparent to those skilled in the art upon areading of the following detailed description when taken in conjunctionwith the drawings wherein there is shown and described an illustrativeembodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] While the specification concludes with claims particularlypointing out and distinctly claiming the subject matter of the presentinvention, it is believed that the invention will be better understoodfrom the following description when taken in conjunction with theaccompanying drawings, wherein:

[0041]FIG. 1 is a block diagram showing the overall process by which adocument can be preserved along with its associated encoded data;

[0042]FIG. 2 is a flow chart illustrating key steps in processing fordocument preservation with associated encoded data;

[0043]FIG. 3 is a visual representation of a data word having multipledata fields, each data field having a predetermined bit depth;

[0044]FIG. 4 is a graph showing a typical relationship of density to thelogarithm of exposure energy for a typical photosensitive medium,indicating separate density ranges of interest;

[0045]FIG. 5 is a visual representation of a mapping operation forcorrelating data fields within a data word of a larger bit depth to datafields within an 8-bit byte;

[0046]FIG. 6 is a visual representation of an 8-bit byte used in amapping operation such as that illustrated in FIG. 5;

[0047]FIG. 7 is a plane view showing one possible layout arrangement fora preserved document record;

[0048]FIG. 8 is a plane view showing a metadata record and calibrationstrip on a media roll;

[0049]FIG. 9 is an example data listing for metadata informationapplicable to a media roll, cassette, or other unit; and

[0050]FIGS. 10a through 10 d show an example structure and data fieldsfor metadata information applicable to a preserved document record.

DETAILED DESCRIPTION OF THE INVENTION

[0051] The present description is directed in particular to elementsforming part of, or cooperating more directly with, apparatus inaccordance with the invention. It is to be understood that elements notspecifically shown or described may take various forms well known tothose skilled in the art.

[0052] Referring to FIG. 1, there is shown a preservation system 80 foraccepting an input document and its associated data, encoding the data,and writing the rasterized image and data encoding onto a monochromepreservation medium to generate a preserved document record 90. Acontrol processing unit 88, typically a computer workstation, accepts aninput document in electronic form from any of a number of possiblesources. One input source could be a networked graphics workstation 82.Alternately, an input document could be from a printed page 84,photograph, or other printed image that can be converted to electronicform by a scanner 86. Other possible document sources could include, butare not limited to, digital camera images, Photo CD images, on-lineimage archives, computer-generated images such as from CAD and graphicsdesign software packages and multimedia software packages, documentprocessing systems, and imaging instruments, for example. Documentscould include data files of many types, including web pages,spreadsheets, email, electronic files from programs such as MicrosoftWord, PowerPoint, Excel, and the like.

[0053] Control processing unit 88 accepts the document data from anysuitable source and formats image data into a rasterized form suitablefor a printer 92. In rasterized form, the document is converted into oneor more images. Each rasterized image comprises a two-dimensional arrayof pixels, with each pixel having an assigned value, such as atristimulus color value, for example. In addition, control processingunit 88 may also format, encode, and rasterize additional data ormetadata to be associated with the document and to be imaged along withthe document onto preserved image record 90. This additional data ormetadata may be provided by software that executes on control processingunit 88 itself or may be provided from graphics workstation 82 or fromsome other data source. This data or metadata could include informationentered by a user or customer of preservation system 80.

[0054] Monochrome Preservation Media for Images and Encoded Data

[0055] Examples of suitable human-readable preservation media forimaging by preservation system 80 include microfilm and related filmproducts and other types of media having similar long-life expectancyand excellent image stability. In addition to film-based media, someother media types that may be acceptable, in some form, for use ashuman-readable preservation media include the following:

[0056] (a) electrophotographic media, when properly treated andfinished;

[0057] (b) thermal media, such as thermal dye sublimation media;

[0058] (c) inkjet media, particularly using plastic film or reflectivematerials; and

[0059] (d) metal plate materials, written using methods such as etchingand laser ablation.

[0060] The materials that are used for human-readable preservation mediaare characterized by exceptionally long useful life. This is in contrastto conventional binary storage media, such as magnetic tapes or disks oroptical storage media. These conventional media types are not readableto the human eye, whether aided by magnification or unaided, and are notsuitable for reliable long-term data storage due to their relativelyshort lifespan and due to hardware and software dependencies for dataaccess from these media. For example, changes to operating system, CPU,or application software can render data that has been recorded on binarystorage media to be unusable. By contrast, data recorded onhuman-readable preservation media can still be interpreted, regardlessof changes to CPU, operating system, or application software.

[0061] Preservation media are typically provided in some form capable ofholding multiple records or frames. Typical formats include roll,cassette, or cartridge format. Preferably, the preservation mediumexhibits a sufficient, controlled dynamic range that allowsrepresentation of many more individual grayscale levels than aredistinguishable to the human eye. The potential excess capability ofhigh-quality monochrome media, such as, for example, KODAK Film SO-240produced by Eastman Kodak Company, Rochester, N.Y., makes it possible toutilize media of this type for encoding, into image pixels, related datathat is associated with that image.

[0062] Stages in Document Processing

[0063] As the above description suggests, any of a number of types ofdata, including metadata, can be encoded for preservation on amonochrome medium along with the rasterized image of a document. A fewof the numerous types of data that might commonly be preserved with animage include color data, audio, measurement, and animation data, forexample. For the purpose of initial description, the processing sequencefor preservation of document data that is described with reference toFIGS. 2 through 6 below uses, as an illustrative example, the encodingand preservation of tristimulus color data associated with an imagedocument. Following the description for this type of encoding, thediscussion of this specification then broadens its scope to encompassmore general cases of encoding of associated data.

[0064] Referring then to the flow chart of FIG. 2, there is shown aprocessing sequence for encoding document data to a monochrome medium.As was described above, an input file in electronic form is provided tothis process; in the preferred embodiment, the input file includes acolor image. A rasterization step 200 formats the input file to arasterized, pixel format, where each pixel has an associated rastervalue. In the preferred embodiment, this raster value is a tristimuluscolor image value using CIELAB color space, with component values oflightness (L*), a-chroma (a*) and b-chroma (b*). A counterinitialization step 202 and a counter increment step 204 are provided toillustrate the mechanics of looping operation for processing each imagepixel. For each pixel, a monochrome word assignment step 206 assigns aword for storing encoded values for grayscale representation. Theassigned monochrome data word has a predetermined bit depth that is afactor of the density resolution of the preservation medium, thedensity-marking characteristics of printer 92, and the performancecharacteristics of an intended scanning device for scanning andextracting encoded data at some future time. The data word is itselfpartitioned into a first data field, a second data field, and possiblethird and subsequent data fields. In the preferred embodiment, themonochrome data word has first, second, and third data fields forencoding lightness, a-chroma, and b-chroma values respectively. For eachpixel an encoding step 208 is then executed. In encoding step 208, thefirst component value, lightness L* in the preferred embodiment, isencoded in a first data field of the monochrome data word. A secondvalue is then encoded in a second data field of the monochrome dataword. This is the a-chroma value a* in the preferred embodiment. Theb-chroma value b* is then encoded in a third data field of themonochrome data word in the preferred embodiment. However, other typesof data could alternately be encoded into the second, third, andsubsequent data fields as part of encoding step 208. A number of datarepresentation schemes can be employed for encoding additional values toadditional data fields of the monochrome data word. At the conclusion ofencoding step 208, a grayscale forming step 210 is then executed. Ingrayscale forming step 210, the various data fields in the monochromedata word are used to generate a grayscale value for imaging the pixel.The monochrome data word can be used without any modification;alternately, its fields can be concatenated or otherwise combined insome other order. In an imaging step 212, then, the pixel can be formedby printer 92 with the intended grayscale value generated in grayscaleforming step 210. Finally, a looping decision step 214 determineswhether or not each pixel has been assigned its grayscale value.

[0065] Those skilled in the computing arts can readily recognize thatthe flow chart of FIG. 2 illustrates only one possible implementation ofimage encoding and printing using a loop, using the mechanics of steps202, 204, and 214. Alternate logic flow sequences could be used. Inpractice, imaging step 212 would most likely write the data for pixelsinto an intermediate memory buffer or similar structure, so that acomplete image could be sent to printer 92 at one time. Regardless ofthe exact processing mechanics, however, the basic assignment and valuemapping scheme outlined in steps 200, 206, 208, and 210 of FIG. 2 wouldbe carried out in some fashion in order to implement the method of thepresent invention.

[0066] As shown in FIG. 3, for most standard tristimulus color imaging,the input file is encoded in a 24-bit raster value 100. A preprocessingstep may be needed to convert color image data into a suitable formatsuch as that represented in FIG. 3. One common color image format usesthe familiar CIE 1976 L*a*b* or CIELAB color space of the CIE,Commission Internationale de l'Eclairage (International Commission onIllumination), well known to those skilled in the color imaging arts.For the CIELAB format, there are three channels of information:lightness (*L), chroma (a*) and chroma (b*). Each channel of informationuses 8-bits, so that a complete 24-bit word is needed to express theCIELAB L*a*b* color space value of each image pixel, as was shown inFIG. 3. Raster value 100 as shown in FIG. 3 has a bit depth of 24-bitswith three data components. A first data component 104 contains the L*channel value. A second data component 106 contains the a* channelvalue. A third data component 108 contains the b* channel value.

[0067] Ideally, it would be advantageous to be able to store each 24-bitCIELAB L*a*b* value for each pixel. However, there are two practicalconsiderations that underlie the implementation of the encoding schemethat follows:

[0068] (1) limitations of the monochrome media. While it may betheoretically possible to accurately reproduce 10-, 12-, 14-bits orgreater resolution on a monochrome medium, existing media and imagingtechniques would make it very difficult to approach the 24-bitresolution that would be needed for full, lossless encoding.

[0069] (2) limitations of human perception. With respect to monochromeimaging, the human eye is sensitive to a limited number of grayscalemonochrome gradations. In practice, as few as 16 different grayscalelevels provide monochrome representations of color images that areconsidered visually accurate and pleasing.

[0070] As the graph of FIG. 4 shows for a typical photosensitive medium,the density response can be segmented into three overall regions. Thehuman eye is most sensitive over a high-contrast region 124. Thephotosensitive medium also exhibits density response over a shoulderregion 122 and a toe region 126, however, human perception is not highlysensitive within these high and low extremes. In conventionaltristimulus color-to-monochrome mapping schemes, only high-contrastregion 124 is used, and typically only for mapping to a correspondinglightness channel value.

[0071] In light of these considerations, then, encoding step 208 of thepresent invention, shown in FIG. 2, performs a mapping from the 24-bitL*a*b* color space representation of raster value 100 to an 8-bit bytethat serves as a monochrome data word 120. Referring to FIG. 5, there isshown the mapping scheme from raster value 100 to monochrome data word120 as used in a preferred embodiment. The 8-bit value in first datacomponent 104, containing the L* value, is mapped to a first data field114, which contains 4-bits. This mapping enables as many as 16 discretegrayscale levels to be represented for the lightness values of pixels inthe original color image. The 8-bit value in second data component 106,containing the a* value, is mapped to a second data field 116, whichcontains 2-bits. Similarly, the 8-bit value in third data component 108,containing the b* value, is mapped to a third data field 118, which alsocontains 2-bits.

[0072] For mapping of components 104, 106, and 108 to data fields 114,116, and 118 respectively, a number of methods can be used. In thepreferred embodiment, mapping is performed using a straightforwardhistogram and statistical techniques for mapping a large set of multiplevalues to a smaller set of representative key values, where each keyvalue allows a reasonable approximation of a set of nearby largervalues. For example, for actual image data values ranging from 18 to 23,a representative key value 20 may be chosen. Further encoding processesmay then map key value 20 to an integer value that can be representedusing 2 or 4-bits. Such statistical and mapping techniques, familiar inthe data processing arts, enable effective “compression” of image dataso that some amount of color data that may have been originally obtainedat 8-bit resolution can be preserved in a 2-bit or 4-bit data field ofmonochrome data word 120.

[0073] In the preferred embodiment, as is shown in FIG. 5, the 2-bitsfor each a* value, and the 2-bits for each b* value in monochrome dataword 120 allow the mapping of corresponding 8-bit chroma values to theappropriate one of the indexed a and b chroma values. Similarly, using4-bits for the L* value allows mapping of an 8-bit lightness value to anappropriate indexed value with higher resolution.

[0074] Returning back to FIG. 2, grayscale forming step 210 may be nomore complicated than simply using, as the grayscale value, all datafields 114, 116, and 118 in monochrome data word 120, plus anyadditional data fields into which monochrome data word 120 ispartitioned. Optionally, depending on the available monochrome densityresolution, customer requirements, or other factors, only individualdata fields 114, 116, and 118 may be used or fields 114, 116, and 118may be concatenated in any suitable combination.

[0075] The procedure of FIG. 2 is executed for all pixels in therasterized document. Note that the monochrome image that prints as aresult of the process described above with reference to FIG. 2 may havethe same overall appearance as a monochrome image produced from a colorimage by using only the lightness L* channel information. However,unlike conventional methods that use a relative lightness value andpreserve no chroma information, the method of the present inventionallows an indexed lightness value to be represented and preserves chromainformation in the lower 4-bits of the 8-bit grayscale value. Since thelower 4-bits are not readily perceptible to the human observer, theinformation stored in these bits does not interfere with the overallappearance of the preserved image, however, scanning the preserved imagewith a high-resolution scanning device will allow the encoding of thelower 4-bits to be retrieved.

[0076] Metadata about the Document

[0077] In addition to pixel grayscale values, there may be moreinformation needed for re-creation of the original full-color image orneeded for accompanying the image itself. Referring to FIG. 7, there isshown an encoded image 96 on preserved document record 90. Below image96 is a document metadata section 94. Document metadata section 94provides, in human-readable form, necessary information for interpretingthe document data in encoded image 96. Information in document metadatasection 94 could include any of the following, for example:

[0078] (a) key values or values that occur most frequently;

[0079] (b) color space parameters or pointers to a color palette;

[0080] (c) metadata on bit and data field assignment for grayscalevalues;

[0081] (d) data field concatenation scheme used; and

[0082] (e) data field mapping scheme used.

[0083] Referring to FIGS. 10a through 10 d, there is shown an example ofthe human-readable data provided in document metadata section 94.

[0084] In general, the metadata fields must be written in human-readableformat. Text characters are typically used for encoding in a data formatthat is open, extensible, and self-defining, such as extensible markuplanguage (XML), for example. This human-readability allows portions ofthe document to be scanned and automatically interpreted, for example,using tools such as optical character recognition (OCR).

[0085]FIG. 10a shows the overall structure of document metadata section94 in a preferred embodiment. Encoded using XML, document metadatasection 94 includes a header section 94 h, followed by color channelsections 94 c 1, 94 c 2, and 94 c 3, one for each L*a*b* color channel.A terminating trailer section 94 t denotes the end of the file formetadata section 94. FIGS. 10b, 10 c and 10 d then show metadata fieldsfor color channel sections 94 c 1, 94 c 2, and 94 c 3 respectively. Eachcolor channel section 94 c 1, 94 c 2 and 94 c 3 gives information on bitpositions used for encoding color channel data, on value ranges, and onmapping definitions for encoding and decoding values. Ellipses ( . . . )indicate where lines have been removed for simplifying and abbreviatingFIGS. 10a and 10 b.

[0086] By way of illustration, FIG. 10b shows how lightness L* valuesfrom 0 to 100 can be mapped to integers from 0 to 15, allowing the L*data to be encoded in a 4-bit data field 114. In the third mappingdefinition given, for example, minimum and maximum boundary values arelisted as follows:

<Channel_Value min=“12”max=“17”>

[0087] Following this boundary value listing, an encoded value from 0-15is defined for the range, as follows:

<Encoded_Value>2</Encoded_Value>

[0088] Then, a value for decoding is provided, showing the value thatwill be assigned, from the original range of 0 to 100, upon decoding ofthe encoded value:

<Decode_Value>12</Decode_Value>

[0089] From this simple, partial illustration, it can be seen that, foran image encoded using this mapping method, values originally in therange 12-17 will be represented as value 12 when the document image isdecoded and restored. There will be some loss of image quality; however,by selecting the mapping ranges carefully, a reasonably closeapproximation of the original document image can be preserved.

[0090] Metadata about the Media

[0091] Referring to FIG. 8, media imaging characteristics must also beprovided in order to decode encoded information from any image 96 on themedia roll 190. In a preferred embodiment, the function of preservingmedia imaging characteristics is performed by assigning one or moreseparate media metadata documents 194 to document positions on mediaroll 190. Note that media roll 190 could be a roll of media or could bea cartridge, cassette, or other packaging unit. Information in mediametadata document 194 could include any of the following, for example:

[0092] (a) media calibration data or look-up tables; and

[0093] (b) error-correction encoding information.

[0094] In order for media metadata document 194 to be useful on anyfuture hardware platform, the encoded data in media metadata document194 must be in human-readable form. Referring to FIG. 9, there is shownan example of a portion of the encoding of media metadata document 194in the preferred embodiment. As shown in FIG. 9, media metadata document194 may include write and read calibration data for the preservationmedium and characteristics for printer 92.

[0095] In addition to the media metadata and image metadata componentslisted above, there can be additional metadata that is associated withthe roll, cartridge, cassette, or other unit in which the preservationmedium is packaged. This metadata can include information on media type,aging characteristics, directory or document tracking data, and otherinformation, for example.

[0096] Referring again to FIG. 8, a calibration patch 196 is alsoprovided as part of the media metadata to allow calibration of a scannerfor reading individual pixels of each image 96. In a preferredembodiment, calibration patch 196 is provided along with metadatasection 194. A number of alternatives are possible, including havingcalibration patch 196 associated with the individual image 96 or with agroup of images 96. Calibration patch 196 could follow a simple format,establishing points along a non-linear density vs. code value curve or,where density is linear with respect to a range of code values,establishing end-points of a line or line segment. Calibration patch 196could alternately include numeric annotation to identify the intendedvalues for one or more densities reproduced in the patch.

[0097] The contone image mapping method described above is somewhatlossy. That is, due to the approximation provided using histograms andstatistical techniques, a color image restored from its preserveddocument record 90 would not exhibit precisely its original colors inall cases. However, extensions of the embodiment described above couldbe used to improve storage for chroma as well as for lightness channels.For example, with 12-bit resolution, data fields 114, 116, and 118 couldbe scaled to 3- or 4-bits, allowing additional gradation in chroma dataas stored. With higher resolution, additional data could be encoded. Themethod of the present invention can be practiced given any reasonablyhigh resolution, with data fields assigned and organized accordingly. Asa general principle, increasingly more robust arrangements are possiblewhen larger bit depths become available.

[0098] Generalized Data Coupling to Document Image

[0099] The example outlined above with reference to FIGS. 2 through 7was directed to the encoding of L*a*b* values in monochrome pixels. Thesame method could alternately be adapted for storing other types ofinformation within grayscale levels, with selected data fields in any ofa number of arrangements. With reference to FIG. 6, for example, thevisual appearance of an image could be preserved using first data field114 for grayscale representation, while using second and third datafields 116 and 118, whether separately or combined, for storage ofalternate information. For example, by combining second and third datafields 116 and 118, monochrome data words 120 for successive pixelscould be used to store a sequence of audio bytes, with each monochromedata word 120, that is, each pixel; storing one half byte.

[0100] The mapping method of the preferred embodiment could be alteredin a number of different ways within the scope of the present invention.For example, it might be desired to arrange fields differently formapping L*a*b* values. In a particular application, there may be noadvantage in printing an image with accurate monochrome representation;in such a case, L* values might be mapped to alternate fields withinmonochrome data word 120. Any arrangement of data fields could be usedas an alternative to the structure shown in FIG. 6. For example, thirddata field 118 or some additional data field could be assigned for imagemetadata, security information, authentication information such as adigital signature, error correction data, information about the overalldocument, or a reference to such information. The data stored in a datafield could be encoded data or could be one part of a byte, word, orother data unit, where the individual parts of the data unit spanmultiple pixels. A data field could store data directly, or store areference or pointer to data, such as a pointer to a color palette, forexample. Fields in addition to data fields 114, 116, and 118 could beassigned, for encoding additional data to be preserved in preserveddocument record 90.

[0101] Encoding Data Using Shadows/Highlights Regions

[0102] Referring back to the density curve of FIG. 4, it is instructiveto observe that images are primarily represented using densities withinhigh contrast region 124. In general, toe region 126, representing verylow densities, and shoulder region 122, representing very high densitiesmay be usable for data storage. This may mean using very dark or verylight pixels within image 96 for storing encoded data, for example,where pixels above or below specific threshold densities are usedprimarily for data encoding.

[0103] Alternate Mapping Schemes

[0104] For preservation of color information, use of the CIELAB L*a*b*format is most favorable, since a lightness channel L* value easily mapsto a corresponding grayscale value. However, data representation formatsother than the tristimulus CIELAB L*a*b* format of the preferredembodiment can be used. For example, color data could be stored inCIELUV format, where tristimulus values represent brightness, hue, andsaturation. Alternately, color data could be encoded in tristimulus RGBformat, cyan, magenta, yellow (CMY) format or in CMYK format (with addedblack component). Or, color data could be encoded in a proprietarytristimulus data format, such as in KODAK Photo YCC Color InterchangeSpace, for example. In order to store all of the component values forthe selected color space, the rasterized data values to be encoded wouldhave a large bit depth, such as 24- or 32-bits in some cases. Monochromedata word 120, however, into which the components of tristimulus andother formats would be encoded, would have a small bit depth, such asthe 8-bit monochrome data word 120 of FIG. 6. The arrangement of fieldswithin monochrome data word 120 can be freely adapted to suit theencoding requirements for color accuracy. As with the L* channelinformation in the example of FIG. 5, it may work best to map onecomponent of color data using relatively more bits. For RGB color data,for example, it may be most effective to map green values to a 4-bitfield, while mapping red and blue values, which may have less impact onsome images, to smaller 2-bit fields. The values used in any field couldbe pointers to other values, such as the L*, a* and b* channel values infirst, second, and third data fields 114, 116, and 118 of FIG. 6. Or,these values could be sufficient in themselves, as might a 4-bit L*channel value stored in first data field 114. Overall, the methods ofthe present invention as disclosed herein could be used for mapping anytype of color representation data format from one data structure toanother.

[0105] Images printed on preserved document record 90 could be positiveor negative, with image density appropriately assigned for thepreservation medium.

[0106] Depending on factors such as image type, spatial resolution, anddata bit depth available due to density resolution, any number ofalternate mapping schemes could be implemented, including the following:

[0107] (a) use of “guard bits.” Deliberate assignment of guard bits asseparators for data fields may help to more clearly distinguish encodeddata values; and

[0108] (b) use of neighboring values and relative offsets. A number ofdata representation schemes can be employed that extrapolate imagevalues for a pixel from those of neighboring pixels or that provide onlyoffsets from an averaged value.

[0109] The invention has been described in detail with particularreference to certain preferred embodiments thereof, but it will beunderstood that variations and modifications can be effected within thescope of the invention. Therefore, what is provided is a method forpreservation of data associated with an image on monochrome media.

[0110] Parts List

[0111]80. Preservation system

[0112]82. Graphics workstation

[0113]84. Printed page

[0114]86. Scanner

[0115]88. Control processing unit

[0116]90. Preserved document record

[0117]92. Printer

[0118]94. Metadata section

[0119]96. Image

[0120]100. Raster value

[0121]104. First data component

[0122]106. Second data component

[0123]108. Third data component

[0124]114. First data field

[0125]116. Second data field

[0126]118. Third data field

[0127]120. Monochrome data word

[0128]122. Shoulder region

[0129]124. High-contrast region

[0130]126. Toe region

[0131]190. Media roll

[0132]194. Media metadata document

[0133]196. Calibration patch

[0134]200. Rasterization step

[0135]202. Counter initialization step

[0136]204. Counter increment step

[0137]206. Monochrome word assignment step

[0138]208. Encoding step

[0139]210. Grayscale forming step

[0140]212. Imaging step

[0141]214. Looping decision step

What is claimed is:
 1. A method of encoding, in a monochrome medium,data about a document that has been received in electronic form, themethod comprising: (a) converting the document to a rasterized image inwhich each pixel is assigned a raster value; (b) for each pixel: (b1)assigning a data word having a predetermined bit depth, said data wordcomprising a first data field and a second data field; (b2) encoding afirst component of said raster value into said first data field; (b3)encoding a second data value into said second data field; (b4)generating a grayscale data value comprising said first data field andsaid second data field; and (b5) forming, onto the monochrome medium, agrayscale pixel conditioned by said grayscale data value.
 2. A method ofencoding as in claim 1 wherein said monochrome medium is aphotosensitive medium.
 3. A method of encoding as in claim 1 wherein thedocument comprises computer generated images.
 4. A method of encoding asin claim 1 wherein the document comprises text.
 5. A method of encodingas in claim 1 wherein the document comprises a slide presentation.
 6. Amethod of encoding as in claim 1 wherein the document comprises a webpage.
 7. A method of encoding as in claim 1 wherein the documentcomprises a spreadsheet.
 8. A method of encoding as in claim 1 whereinsaid raster value comprises lightness and chroma data components.
 9. Amethod of encoding as in claim 1 wherein said raster value comprisesred, green, and blue data components.
 10. A method of encoding as inclaim 1 wherein said raster value comprises cyan, magenta, and yellowdata components.
 11. A method of encoding as in claim 1 wherein saidraster value comprises hue, saturation, and lightness data components.12. A method for encoding as in claim 1 wherein said second data valueis lightness or chroma data.
 13. A method for encoding as in claim 1wherein said second data value is red, green, or blue data.
 14. A methodfor encoding as in claim 1 wherein said second data value is cyan,magenta, or yellow data.
 15. A method for encoding as in claim 1 whereinsaid second data value is hue, saturation, or lightness data.
 16. Amethod of encoding as in claim 1 wherein said second data valuecomprises metadata about the document.
 17. A method of encoding as inclaim 1 wherein said second data value comprises encoded audio data. 18.A method of encoding as in claim 1 wherein said second data valuecomprises authentication data about the document.
 19. A method ofencoding as in claim 1 wherein said second data value comprisesanimation data about the document.
 20. A method of encoding as in claim1 wherein said second data value comprises security information aboutthe document.
 21. A method of encoding as in claim 1 wherein said seconddata value comprises information about data mapping.
 22. A method ofencoding as in claim 1 wherein said second data value comprises areference to other information about the document.
 23. A method ofencoding as in claim 1 wherein said second data value comprises apointer to a color palette.
 24. A method of encoding as in claim 1wherein said second data value comprises measurement data.
 25. A methodof encoding as in claim 1 wherein the step of encoding a first componentof said raster value into said first data field is conditioned by astatistical frequency.
 26. A method of encoding as in claim 1 furthercomprising the step of writing, onto said monochrome medium, ahuman-readable metadata record that comprises information for decodingsaid data about the document.
 27. The method of encoding as in claim 1wherein said second data value comprises error correction information.28. The method of encoding as in claim 26 wherein said human-readablemetadata record is in XML format.
 29. The method of encoding as in claim26 wherein said human-readable metadata record is readable by an opticalcharacter recognition apparatus.
 30. A method of encoding, in amonochrome medium, data about a document that has been received as arasterized image, in which each pixel is assigned a raster value, themethod comprising, for each pixel: (a) assigning a data word having apredetermined bit depth, said data word comprising a first data fieldand a second data field; (b) encoding a first component of said rastervalue into said first data field; (c) encoding a second data value intosaid second data field; (d) generating a grayscale data value comprisingsaid first data field and said second data field; and (e) forming, ontothe monochrome medium, a grayscale pixel conditioned by said grayscaledata value.
 31. A method of encoding as in claim 30 wherein saidmonochrome medium is a photosensitive medium.
 32. A method of encodingas in claim 30 wherein the document comprises computer generated images.33. A method of encoding as in claim 30 wherein the document comprises ascanned image.
 34. A method of encoding as in claim 30 wherein thedocument comprises a digital camera image.
 35. A method of encoding asin claim 30 wherein said raster value comprises lightness and chromadata components.
 36. A method of encoding as in claim 30 wherein saidraster value comprises red, green, and blue data components.
 37. Amethod of encoding as in claim 30 wherein said raster value comprisescyan, magenta, and yellow data components.
 38. A method of encoding asin claim 30 wherein said raster value comprises hue, saturation, andlightness data components.
 39. A method for encoding as in claim 30wherein said second data value is lightness or chroma data.
 40. A methodfor encoding as in claim 30 wherein said second data value is red,green, or blue data.
 41. A method for encoding as in claim 30 whereinsaid second data value is cyan, magenta, or yellow data.
 42. A methodfor encoding as in claim 30 wherein said second data value is hue,saturation, or lightness data.
 43. A method of encoding as in claim 30wherein said second data value comprises metadata about the document.44. A method of encoding as in claim 30 wherein said second data valuecomprises encoded audio data.
 45. A method of encoding as in claim 30wherein said second data value comprises authentication data about thedocument.
 46. A method of encoding as in claim 30 wherein said seconddata value comprises animation data about the document.
 47. A method ofencoding as in claim 30 wherein said second data value comprisessecurity information about the document.
 48. A method of encoding as inclaim 30 wherein said second data value comprises information about datamapping.
 49. A method of encoding as in claim 30 wherein said seconddata value comprises a reference to other information about thedocument.
 50. A method of encoding as in claim 30 wherein said seconddata value comprises a pointer to a color palette.
 51. A method ofencoding as in claim 30 wherein said second data value comprisesmeasurement data.
 52. A method of encoding as in claim 30 wherein thestep of encoding a first component of said raster value into said firstdata field is conditioned by a statistical frequency.
 53. A method ofencoding as in claim 30 further comprising the step of writing, ontosaid monochrome medium, a human-readable metadata record that comprisesinformation for decoding said data about the document.
 54. The method ofencoding as in claim 1 wherein said second data value comprises errorcorrection information.
 55. The method of encoding as in claim 26wherein said human-readable metadata record is in XML format.
 56. Themethod of encoding as in claim 26 wherein said human-readable metadatarecord is readable by an optical character recognition apparatus.
 57. Amethod of storing, on a monochrome medium, a tristimulus color imagevalue associated with each pixel in a color document, wherein eachtristimulus color image value comprises a first data value, a seconddata value, and a third data value, the method comprising: (a) assigninga data word to said each pixel, said data word having a predeterminedbit depth for storing a grayscale value, said data word comprising afirst field, a second field, and a third field; (b) encoding, for eachsaid pixel, said first data value in said first field, said second datavalue in said second field, and said third data value in said thirdfield, thereby forming said grayscale data value in said data word; and(c) forming a grayscale image onto the monochrome medium, wherein thedensity of said each pixel corresponds to said grayscale data value insaid data word for each said pixel.
 58. The method of storing atristimulus color value as in claim 57 wherein said first data value isa lightness value, said second data value is an a-channel chroma value,and said third data value is a b-channel chroma value.
 59. The method ofstoring a tristimulus color value as in claim 57 wherein said first datavalue is a brightness value, said second data value is a hue value, andsaid third data value is a saturation value.
 60. The method of storing atristimulus color value as in claim 57 wherein said first data value isa red value, said second data value is a green value, and said thirddata value is a blue value.
 61. The method of storing a tristimuluscolor value as in claim 57 wherein said first data value is a cyanvalue, said second data value is a magenta value, and said third datavalue is a yellow value.
 62. The method of storing a tristimulus colorimage value as in claim 57 wherein the step of encoding said first datavalue in said first field is conditioned by the statistical frequency ofsaid first data value in said color image.
 63. A method of storing, on amonochrome medium, a CMYK color image value associated with each pixelin a color document, wherein each CMYK color image value comprises afirst data value, a second data value, a third data value, and a fourthdata value, the method comprising: (a) assigning a data word to saideach pixel, said data word having a predetermined bit depth for storinga grayscale value, said data word comprising a first field, a secondfield, a third field and a fourth field; (b) encoding, for each saidpixel, said first data value in said first field, said second data valuein said second field, said third data value in said third field, andsaid fourth data value in said fourth field, thereby forming saidgrayscale data value in said data word; and (c) forming a grayscaleimage onto the monochrome medium, wherein the density of said each pixelcorresponds to said grayscale data value in said data word for each saidpixel.